In recent years, an EER (Envelope Elimination and Restoration) system has been used as one of the amplification systems used for power-amplifying a high-frequency modulation signal in a base station of a cellular phone, etc. The EER system amplifies a signal by extracting an amplitude component (envelope) and phase component from a modulation signal to be amplified, and modulating the amplitude of a signal corresponding to the phase component with a signal corresponding to the amplitude component, so that the amplitude of the modulated signal becomes proportional to the amplitude of the original modulation signal.
More specifically, voltage that follows the extracted envelope is applied to a saturated amplifier as a power supply voltage. This amplifier causes the amplitude of the amplified signal to follow the extracted envelope, by amplifying the signal corresponding to the phase component. The voltage that follows the envelope is obtained by, for example, power-amplifying a detected signal (referred to as “envelope signal” hereinafter) obtained through envelope detection performed on the modulation signal to be amplified. The saturated amplifier is used to power-amplify the envelope signal in order to achieve high efficiency. For example, the envelope signal is demodulated as a modulation signal, by switching elements with a PWM signal, which is generated by performing pulse width modulation on the envelope signal, and integrating the PWM signal amplified as a result of switching.
In most cases, a class D amplifier having complementary switching elements push-pull connected to each other, or a class E amplifier for turning on a switching element when voltage applied from an inductance element is zero, is used in order to amplify a PWM signal. However, it is technically difficult for the class D amplifier to enhance the withstand voltages of the complementary switching elements in a balanced manner. In the class E amplifier, surge voltage that is applied from the inductance element to the switching element when the switching element is turned off may exceed power supply voltage significantly under some design and operating conditions. For these reasons, there are, inevitably, limits in applying the push-pull switching elements or the single switching element to a high-frequency and high-power amplifier.
Additionally, the abovementioned PWM signal contains an envelope signal component of relatively low frequency and a PWM signal component of high frequency. A PWM signal amplifier is therefore required to have wideband frequency characteristics. As such an amplifier that meets the requirement, it is conceivable that the distributed amplifier disclosed in Patent Literature 1, for example, is applied to a PWM signal amplifier.